The present invention relates to a catalytic combustion heater that heats fluid to be heated, which is a liquid or gas.
A so-called catalytic combustion heater, which causes an oxidation reaction of an flammable gas (fuel gas) with a catalyst and heats a fluid to be heated with the generated heat, is known, and various applications of the heater, such as home use and vehicular use, have been studied (e.g., Japanese Unexamined Patent Publication (KOKAI) No. Hei 5-223201).
A catalytic combustion heater has a catalyst-carrying heat exchanger having, in a flow passage of an flammable gas, tubes where an object fluid to be heated, which is a liquid or gas, flows, and multiple catalyst-carrying fins are integrally joined to the outer surfaces of the tubes. An oxidation catalyst, such as platinum or palladium is used for the multiple fins.
When the catalyst-carrying fins are heated to or above an activation temperature and contact the flammable gas, an oxidation reaction occurs on the surfaces of the fins. The oxidation reaction heat generated at that time is transferred from the fins into the tubes, thereby heating the object fluid that flows in the tubes.
The flammable gas is mixed with a combustion support gas (normally, air) for oxidizing the flammable gas, and the mixed gas is supplied as a fuel gas into the catalyst-carrying heat exchanger. The catalyst-oriented oxidation reaction occurs in widely varying range of the flammable gas concentration. Therefore, unburned gas that has not reacted upstream can be burned with a catalyst on the downstream side, and combustion can be carried out in the entire heat exchanger. This provides a compact and high-performance heater as compared with burner type heaters, which have been typical so far.
There is a type in which the direction of the flow of the flammable gas in a catalyst-carrying heat exchanger is opposite to the direction of the flow of the object fluid. In this case, as the slope of the concentration of the flammable gas coincides with the slope of the temperature of the object fluid, the heat exchanging efficiency can be improved. That is, since an inlet port for the object fluid is provided near the outlet of the fuel-gas flow passage, the heat of the exhaust gas can heat the object fluid efficiently by making the combustion exhaust gas, immediately before being discharged, contact the tubes where the cooler object fluid flows.
The feed rate of the combustion support gas is normally set in a range of about 1 to 5 times the amount necessary for oxidation. To improve the heat exchanging efficiency, it is preferred to reduce the flow rate of exhaust gas by making the feed rate as small as possible to thereby limit the dumping of the generated heat, unused, with the exhaust gas.
However, the combustion exhaust gas contains a considerable amount of vapor produced by the oxidation reaction, so that when the temperature of the combustion exhaust gas drops, the vapor may condense into droplets.
In the construction in which the direction of the flow of the flammable gas in a catalyst-carrying heat exchanger is opposite to the direction of the flow of the object fluid, particularly, the cooler object fluid is supplied near the outlet for the combustion exhaust gas as mentioned above. Therefore, vapor may condense on the surfaces of the low-temperature tubes and the surfaces of the fins that are integral to the tubes and wet the surface of the oxidation catalyst. In this case, there is a problem in that the oxidation catalyst becomes inactive, thus interfering with the oxidation reaction and causing unburned gas to be discharged.
If the feed rate of the combustion support gas is low, it becomes easier for the temperature of the catalyst to rise and the non-uniform distribution of the fuel gas may cause the catalyst temperature to exceed the combustion point (570xc2x0 C. for hydrogen fuel) at the location where high-concentration flammable gas is supplied or the location where the object fluid does not flow smoothly, thus generating a flame. When a flame is produced, the catalyst may have heat deterioration (normally, the deterioration occurs at or above 700xc2x0 C.), which lowers the catalytic performance. Because the catalyst reaction is caused in the entire heat exchanger as mentioned above, however, it is difficult to specify where a flame will be produced and it is hard to detect the flame.
According to the above conventional catalytic combustion heater, however, if the catalyst on the upstream side of the fuel-gas flow passage is not sufficiently active at the time the heater is activated, unreacted fuel gas (unburned fuel gas) may be discharged or keep flowing downstream and become a high-concentration fuel gas, which may contact the oxidation catalyst in the vicinity of the outlet of the fuel-gas flow passage and may spontaneously react with it and cause a fire. One way to prevent this is to gradually raise the temperature of the tubes and fins at the individual portions of the fuel-gas flow passage while monitoring those temperatures. This method complicates the structure and extends the activation time longer.
Further, there are expected applications of a catalytic combustion heater, which burns an flammable fuel gas using an oxidation catalyst and heats an object fluid using the generated heat, such as home use and vehicular use. In such a catalytic combustion heater, a combustion support gas is supplied from one of the open ends of a cylindrical housing having openings at both ends and a fuel-gas feeding section injects the fuel gas from an injection port formed inside the housing, thereby producing a flow of the mixture of the fuel gas and the combustion support gas in the housing. Tubes in which an object fluid to be heated, such as water, flows are located in the housing, and a catalyst section, such as fins carrying an oxidation catalyst, is formed on the outer surfaces of the tubes, thus constituting a catalyst-carrying heat exchanger. The fuel gas that contacts the catalyst section causes an oxidation reaction there, thus causing catalyst combustion. The combustion heat caused by the catalytic combustion is received by the object fluid through the walls of the tubes and is used for heating.
Further, when the combustion output becomes high, a flame is produced, resulting in vapor phase combustion. Since the vapor phase combustion has a higher combustion temperature than the catalytic combustion, it deteriorates the heater, which causes problems such as reducing heat exchanging efficiency and lowering the heating performance. There is a model that has a temperature sensor provided in the catalyst section to detect a temperature rise in the catalyst section from which vapor phase combustion is detected. Even when vapor phase combustion occurs, the detected temperature does not necessarily rise to a level that is considered abnormal unless the temperature sensor is exposed to a flame. When a very small part of the catalyst becomes abnormally hot and a flame is locally produced, therefore, occurrence of vapor phase combustion cannot be detected. In addition, since a threshold value for the detected temperature for determining if vapor phase combustion has occurred is naturally set higher than the temperature of the catalyst section at the time of normal catalytic combustion, it is not possible to detect the occurrence of vapor phase combustion with sufficient precision.
In view of the above problems, it is an object of the present invention to provide a catalytic combustion heater that prevents the activation of an oxidation catalyst from being lowered by condensation of vapor, prevents the catalyst from being deteriorated by the occurrence of a flame, demonstrates sufficient catalytic performance, has excellent heat exchanging efficiency and is safe and highly reliable.
In view of the above problems, it is another object of the present invention to provide a safe and quick-activating catalytic combustion heater that can activate the whole catalyst-carrying heat exchanger quickly with a simple structure while preventing discharge of unburned gas and a fire.
In view of the above problems, it is a further object of the present invention to provide a catalytic combustion heater that can detect the occurrence of vapor phase combustion with high precision.
A catalytic combustion heater according to the present invention includes a catalyst-carrying heat exchanger. The heat exchanger has a fuel-gas flow passage, in which a fuel gas flows. The fuel gas includes a flammable gas and a combustion support gas. Tubes, in which an object fluid to be heated flows, are located within the fuel-gas flow passage. An oxidation catalyst, which is provided on outer surfaces of the tubes, causes an oxidation reaction when the fuel gas contacts the outer surfaces. The catalyst-carrying heat exchanger heats the object fluid with the oxidation reaction heat of the fuel gas. Further included is a detecting section for detecting whether or not the temperature of a combustion exhaust gas in the fuel-gas flow passage has reached its dew-point temperature. Further included is a control section for controlling at least one of the feed rate of the combustion support gas and that of the flammable gas supplied to the fuel-gas flow passage, based on a result of detection by the detecting section.
The detecting section is one of a temperature detecting section for detecting the temperature of the combustion exhaust gas and a temperature detecting section for detecting temperatures-of the outer surfaces of the tubes.
The detecting section is provided in the vicinity of an outlet of the fuel-gas flow passage.
The oxidation catalyst is carried by fins joined to the outer surface of the tubes and the temperature detecting section for detecting the temperatures of the outer surfaces of the tubes is a surface temperature detecting section for detecting surface temperatures of the fins in the vicinity of an outlet of the fuel-gas flow passage.
When the detecting section outputs a detection result such that the temperature of the combustion exhaust gas in the fuel-gas flow passage is equal to or lower than a dew-point temperature, which is determined by the composition of the fuel gas to be supplied, the control section performs control to increase the feed rate of the combustion support gas to raise the temperature of the combustion exhaust gas to or above the dew-point temperature.
When the detecting section outputs a detection result indicating that the temperature of the combustion exhaust gas in the fuel-gas flow passage is equal to or lower than a dew-point temperature, which is determined by the composition of the supplied fuel gas, the control section increases the feed rate of the flammable gas to a downstream part of the fuel-gas flow passage to raise the temperature of the combustion exhaust gas to or above the dew-point temperature.
The catalytic combustion heater further includes an flammable-gas feeding section having a plurality of flammable-gas feed ports, for distributing the flammable gas to an upstream part and a downstream part of the fuel-gas flow passage, and a valve member, which is located in the flammable-gas feeding section, for regulating the flow rate of the flammable gas supplied to the downstream side of the fuel-gas flow passage, and the control section adjusts the position of the valve member.
The flow direction of the fuel gas is opposite to the flow direction of the object fluid.
The combustion support gas is air.
Another catalytic combustion heater according to the present invention includes a catalyst-carrying heat exchanger. The heat exchanger has a fuel-gas flow passage, in which a fuel gas flows. The fuel gas includes a flammable gas and a combustion support gas. Tubes, in which an object fluid to be heated flows, are located within the fuel-gas flow passage. An oxidation catalyst, which is provided on outer surfaces of the tubes, causes an oxidation reaction when the fuel gas contacts the outer surfaces. The catalyst-carrying heat exchanger heats the object fluid with the oxidation reaction heat of the fuel gas. Further included is a detecting section for detecting the concentration of nitrogen oxide contained in the combustion exhaust gas in the fuel-gas flow passage and a control section for controlling at least one of the feed rate of the combustion support gas and that of the flammable gas supplied to the fuel-gas flow passage, based on a result of detection by the detecting section.
In this catalytic combustion heater according to the present invention, the detecting section is provided in the vicinity of an outlet of the fuel-gas flow passage.
In the another catalytic combustion heater according to the present invention, when the detecting section detects that the concentration of the nitrogen oxide is equal to or higher than a given value, the control section decreases the feed rate of the flammable gas or increases the feed rate of the combustion support gas.
A further catalytic combustion heater according to the present invention includes a catalyst-carrying heat exchanger. The heat exchanger has a fuel-gas flow passage, in which a fuel gas flows. The fuel gas includes a flammable gas and a combustion support gas. Tubes, in which an object fluid to be heated flows, are located within the fuel-gas flow passage. An oxidation catalyst, which is provided on outer surfaces of the tubes, causes an oxidation reaction when the fuel gas contacts the outer surfaces. The catalyst-carrying heat exchanger heats the object fluid with the oxidation reaction heat of the fuel gas. Further included is a plurality of flammable-gas feeding passages with different passage resistances for distributing the flammable gas to an upstream part and downstream part of the fuel-gas flow passage, whereby the passage resistances of the plurality of flammable-gas feeding passages are such that when an amount of heat generated in a downstream part of the fuel-gas flow passage is a minimum output of the catalytic combustion heater, the temperature of combustion exhaust gas in the fuel-gas flow passage becomes equal to or higher than a dew-point temperature that is determined by the composition of the fuel gas.
A different catalytic combustion heater according to the present invention includes a catalyst-carrying heat exchanger. The heat exchanger has a fuel-gas flow passage, in which a fuel gas flows. The fuel gas includes a flammable gas and a combustion support gas. Tubes, in which an object fluid to be heated flows, are located within the fuel-gas flow passage. An oxidation catalyst, which is provided on outer surfaces of the tubes, causes an oxidation reaction when the fuel gas contacts the outer surfaces. The catalyst-carrying heat exchanger heats the object fluid with the oxidation reaction heat of the fuel gas. Further included is a detecting section for detecting the temperature of combustion exhaust gas or the concentration of the flammable gas in the vicinity of an outlet of the fuel-gas flow passage and a flow-rate control section for controlling the flow rate of the flammable gas based on the result of a detection of the detecting section.
In this catalytic combustion heater, according to the present invention, the flow-rate control section makes the flow rate of the flammable gas less than that of the combustion support gas until the temperature of the combustion exhaust gas detected by the detecting section exceeds a predetermined temperature or until the concentration of the flammable gas becomes lower than a predetermined concentration. The flow-rate control section increases the flow rate of the flammable gas to a predetermined level when the temperature of the combustion exhaust gas exceeds the predetermined temperature or when the concentration of the flammable gas becomes lower than the predetermined concentration.
In this catalytic combustion heater, according to the present invention, the catalyst-carrying heat exchanger has a fuel distributing section for distributing the flammable gas, the amount of which corresponds to a state of the object fluid flowing in the tubes to individual parts of the tubes.
A still different catalytic combustion heater according to the present invention includes a cylindrical housing having openings at both ends, and a combustion support gas is supplied from one of the open ends. Also included is a fuel-gas feeding section for feeding fuel gas into the housing from an injection port, which is formed toward inside the housing. Included is a catalyst-carrying heat exchanger having a plurality of tubes. The heat exchanger is provided downstream of the injection port in the housing. An object fluid, which is to be heated flows in the tubes. A catalyst section, which is formed on outer surfaces of the tubes, causes an oxidation reaction when contacting the fuel gas. A temperature detecting section provided in the housing in the vicinity of the injection port and closer to the above-mentioned open end than the tubes.
In this catalytic combustion heater according to the present invention, the temperature detecting section is provided on a projection of the fuel-gas feeding section protruding into the housing.